This invention relates to a charge transfer image sensor and more particularly to an image sensor which can produce a video signal having a good picture quality even when very intense light beams are received from a foreground subject.
Generally, an image sensor comprises a photosensing subsection, which is provided with a plurality of photosensing regions. A charge packet stored in the photosensing region constitutes a picture element. Where a bright incoming light beam happens to produce too large an electric charge to be stored in a photosensing region, then such excess electric charge flows into an adjacent photosensing region or into a shift register designed to read out an output from an adjacent sensing region or charge packet, thus preventing said shift register from sensing forth a good video signal. Such event is known as the blooming phenomenon. An image sensor arranged as shown in FIG. 1 is already known as means for suppressing the blooming phenomenon. For briefness of description, FIG. 1 indicates an image sensor constituting five picture elements. A photosensing subsection 1 consists of a plurality of photodiodes 1a to 1e, each converting a photosignal into a signal charge and storing the signal charge. Signal charges stored in the photodiodes 1a to 1e are transferred to a signal-readout section 2 (such as a shift register) of, for example, a 2-phase driving type through a signal charge packet transfer gate subsection 3 comprising five signal charge transfer regions provided with electrodes 3a to 3e respectively. The signal-readout section 2 sends forth an output signal O.sub.1 (video signal). The aforesaid excess charges are conducted to an overflow drain-section 4 through an overflow gate subsection 5 comprising five overflow gate regions provided with electrodes 5a to 5e respectively to be drawn off to the outside. The overflow drain-section 4, signal-readout section 2, overflow gate subsection 5, photosensing subsection 1 and transfer gate subsection 3 are all integrated on, for example, a P-type semiconductor substrate.
FIGS. 2A to 2D show the waveforms of signals supplied to the respective sections and subsections of FIG. 1. FIG. 2E indicates the waveform of an output signal O.sub.1 from the signal-readout section 2 (for example, a shift register). The electrodes 3a to 3e of the respective regions of the signal charge transfer gate subsection 3 are supplied with a signal charge transfer pulse of FIG. 2A at an interval S of integration time, (a total integration period during which light beams are received). The terminals 6a, 6b of the signal-readout section 2 are supplied with clock pulses of FIGS. 2B and 2C. The electrodes 5a to 5e of the overflow gate subsection 5 are impressed with a D.C. voltage of FIG. 2D. FIG. 2E shows the waveform of an output signal O.sub.1 from the signal-readout section 2 which is produced during one scanning period thereof.
At time t.sub.1, the electrodes 3a to 3e are supplied with a signal charge transfer pulse of FIG. 2A. Before the time t.sub.1, signal charges stored in the photodiodes 1a to 1e are delivered to the signal-readout section 2. When scanned, this section produces an output signal O.sub.1 of FIG. 2E. After the time t.sub.1, a signal charge corresponding to the intensity of an incoming light beam is stored in the photodiodes 1a to 1e only during the interval S. The stored signal charge is transfered to the signal-readout section 2 at a time t.sub.2 at which the succeeding transfer pulse of FIG. 2A is received. The regions under the electrodes 5a to 5e impressed with a D.C. voltage of FIG. 2D have a low potential barrier. In contrast, the electrodes 3a to 3e are impressed with a voltage of OV at times other than that at which the signal charge transfer pulse of FIG. 2A is generated. Accordingly, the regions under these electrodes 3a to 3e have a high potential barrier. When, therefore, a signal charge being stored in the photodiodes 1a to 1e during the intergration interval S exceeds a prescribed amount, then said excess portion of a charge flows into the overflow drain section 4 through the regions under the electrodes 5a to 5e to be drawn off to the outside. Thus, a surplus charge produced in a photosensing region which exceeds a prescribed amount is prevented from flowing into an adjacent picture element region or signal-readout section 2, thereby reducing the possibility of the blooming phenomenon being produced. The prior art charge transfer image sensor is indeed effective to minimize the blooming phenomenon, where intense light beams are brought to part of the photo-sensing subsection 1. But if the whole photosensing subsection 1 of the prior art image sensor is exposed to intense light beams, then the photosensing regions are saturated alike, preventing the signal-readout section 2 from producing a good video signal. If, in such case, the integration interval S is shortened, then it is possible to generate a good video signal. However, the shortening of the interval S means the compression of a time base indicated in FIG. 2A. In the above-mentioned case, the time base of the signals of FIGS. 2B, 2C and 2E has to be compressed. This is tantamount to the fact that elements of the driving circuit of an image sensor and a signal-processing circuit are demanded to be the type of high speed operation and consequently high cost, and moreover power consumption is increased.
Where the overflow gate subsection 5 of the prior art image sensor of FIG. 1 is supplied with a pulse ahead of the pulse of FIG. 2A instead of a D.C. voltage, then the period during which the image sensor displays effective photosensitivity can be shortened without increasing the frequency of a clock signal used. The reason is that when a pulse is supplied to the overflow gate subsection 5, a charge stored in the photosensing subsection 1 up to this point is drawn off to the drain section 4. According to this process, however, a signal charge retained in the photodiodes are governed by the voltage of a pulse impressed on the overflow gate subsection 5 and signal charge transfer gate subsection 3, presenting difficulties in producing a good video signal.
It is accordingly an object of this invention to provide a charge transfer image sensor which produces a good video signal regardless of the intensity of light beams irradiated on the photosensing subsection.
Another object of the invention is to provide a charge transfer image sensor which produces a good video signal without increasing the frequency of pulses supplied to the driving circuit and signal-processing circuit over the case of the prior art image sensor.